The field of the invention is feedback circuits employed in motor control systems, and particularly, circuits for providing current feedback signals.
There are numerous control systems in which output current is sensed to form a feedback signal that is employed at the input of the system. For example, in electric motor drives accurate measurement of the instantaneous stator current of the motor may be required to control the high frequency pulse width modulation of the motor voltage to achieve control of the stator current. The successful operation of the motor control system may depend in such case on the accuracy of both the a.c. and d.c. components current feedback signal.
One common means for providing a current feedback signal is to employ a current transformer having its primary winding connected to conduct the current being measured. The disadvantage of using current transformers is that they do not respond to direct current. Another common current sensor is a Hall effect device in which the magnetic field produced by the current is sensed and is employed to generate the feedback signal. The disadvantage with Hall effect devices is that they lack gain stability, and give rise to d.c. offset errors. In addition Hall effect devices often have relatively low sensitivity. Although these two prior solutions are satisfactory in some applications, where high performance control systems are employed the lack of current transformer response to d.c. and the gain instability and insensitivity of Hall effects sensors limits the use of these prior devices in many applications.
Another solution is to measure the voltage drop across a resistor connected to conduct the current. Unfortunately, the current sensing resistor in many applications is connected at a point of high d.c. or a.c. voltage which must be isolated from the control system circuitry to which the current feedback signal is applied. This requires the use of a level shifting circuit which may introduce d.c. offset errors or common mode errors. Also, such a solution does not provide d.c. isolation between the control circuitry and the high voltage circuitry.